Age-related differences in performance on speeded tasks (e.g., simple reaction time, analogical reasoning) are ubiquitous and large. The proposed research is designed to distinguish between the two principal explanations for these age differences: (1) the differences are due to central mechanisms but are epiphenomenal in the sense that differences in speed are readily derived from other psychological mechanisms (e.g., due to younger children's use of less efficient strategies for task solution); and (2) the age differences are central and reflect the operation of some fundamental parameter of the child's cognitive system (e.g., age differences in processing resources). Each of the proposed experiments involves three tasks: mental rotation, visual search, and memory search. In mental rotation, subjects decide if pairs of letters presented in different orientations are identical or mirror images. In the visual search task, subjects are first shown a target letter. Then they are shown an array of letters (e.g., A R W C F) and are asked to determine, as rapidly as possible, if the target letter is in the series. The memory search task is essentially the visual search task in reverse: Subjects are shown the series of letters, then determine if a letter presented subsequently was member of the set. The subjects will range from 8 to 21 years of age. The aim of Experiment 1 is to determine if there are age differences in rates of mental rotation, visual search, and memory search after subjects have extensive practice (to the point that additional practice no longer improves performance). The aim of Experiments 2 and 3 is to determine how speed and accuracy tradeoff in performance, so that processing speeds for different age groups can be based on equivalent levels of accuracy. Finally, if age differences in speed of processing reflect developmental change in a central mechanism, the shape of the growth function for processing speed should be the same across tasks. Thus, the aim of Experiment 4 is to determine if different processing parameters have similar growth functions.